1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging lithographic printing members on- or off-press using digitally controlled laser output.
2. Description of the Related Art
Various methods and technologies exist for encoding documents digitally and transferring the digital representations to output devices. At the encoding stage, these range from hobbyist scanners and associated software to elaborate prepress systems. These systems have replaced traditional "cut and paste" approaches to layout, which required painstaking manual arrangement of the various document components--text, graphic patterns and photographic images--onto a white board for subsequent reproduction. Instead, designers can now manipulate all of these components at once using computers.
Output of the digitally encoded documents can take numerous forms, ranging from laser printing to digital exposure of photographic films to transfer of the image to lithographic plates for subsequent mass-quantity printing. In the latter case, the image to be printed is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
Lithographic plates can be fabricated in various ways, ranging, once again, from traditional manual techniques involving photoexposure and chemical development to automated procedures involving computer control. Computer-to-plate systems can utilize pulses of electromagnetic radiation, produced by one or more laser or non-laser sources, to create physical or chemical changes at selected points of sensitized plate blanks (which, depending on the system, may be used immediately or following conventional photodevelopment); ink-jet equipment used to selectively deposit ink-repellent or ink-accepting spots on plate blanks; or spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to alter the characteristics of certain areas on a printing surface, thereby creating "dots" which collectively form a desired image. As used herein, the term "imaging device" includes radiation sources (e.g., lasers), ink-jet sources, electrodes and other known means of producing image spots on blank printing plates, and the term "discharge" means the image-forming emissions produced by these devices. The term "image" refers to a lithographic representation of the final document to be reproduced. The term "plate" refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution; suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
A second approach to laser imaging involves the use of transfer materials. See, e.g., U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,245,003; 4,395,946; 4,588,674; and 4,711,834. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. During operation the transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Typically, the transfer material exhibits a high degree of absorbence for imaging laser radiation, and ablates--that is, virtually explodes into a cloud of gas and charred debris--in response to a laser pulse. This action, which may be further enhanced by self-oxidation (as in the case, for example, of nitrocellulose materials), ensures complete removal of the transfer material from its carrier. Material that survives ablation adheres to the acceptor sheet.
Alternatively, instead of laser activation, transfer of the thermal material can be accomplished through direct contact. U.S. Pat. No. 4,846,065, for example, describes the use of a digitally controlled pressing head to transfer oleophilic material to an image carrier.
To create a printing plate, the transfer and acceptor materials are chosen to exhibit different affinities for fountain solution and/or ink, so that removal of the transparent layer together with unirradiated transfer material leaves a suitably imaged, finished plate.
Another important application of transfer materials is proofing. Graphic-arts practitioners use color proofing sheets (or simply "color proofs") to correct separation images prior to producing final separation plates, as well as to evaluate the color quality that will be obtained during the printing process. In typical printing processes, multicolor images cannot be printed directly using a single printing plate. Rather, composite color images are first decomposed into a set of constituent color components, or "separations", each of which serve as the basis for an individual plate. The colors into which the multicolor image is decomposed depends on the particular "color model" chosen by the practitioner. The most common color model is based on cyan, magenta, yellow and black constituents, and is referred to as the "CMYK" color model. If the separation is performed properly, subtractive combination of the individual separations produces the original composite image. A color proof represents, and permits the practitioner to view, the final image as it will appear when printed.
A color proof may be produced by irradiative transfer of a coloring agent, corresponding to one of the separation colors, onto a transparent acceptor sheet according to the distribution of that color in the final image. Transfer sheets corresponding to each color of the model can be applied to a single acceptor sheet and sequentially imaged, producing a single-sheet proof. Alternatively, a set of color proofs each corresponding to one of the colors may be superposed on each other in registration, thereby revealing the final image.
Mechanically, laser-based imaging systems can take a variety of forms. Laser output may be provided directly to the surface of a substrate via lenses or other beam-guiding components, or transmitted to the surface from a remotely sited laser using a fiber-optic cable. A controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the substrate surface, scans the output over the surface, and activates the laser at positions adjacent selected points or areas of the substrate. The controller responds to incoming image signals corresponding to the original document or picture being copied onto the substrate to produce a precise negative or positive image of that original. The image signals are stored as a bitmap data file on a computer. Such files may be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept input data in page-description language, which defines all of the features required to be transferred onto the substrate, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the substrate mounted to the interior or exterior cylindrical surface of the drum. In the case of lithographic printing, the exterior drum design is more appropriate to use in situ, on a lithographic press, in which case the print cylinder itself constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between the laser beam and the substrate is achieved by rotating the drum (and the substrate secured thereon) about its axis and moving the beam parallel to the rotation axis, thereby scanning the substrate circumferentially so the image "grows" in the axial direction. Alternatively, the beam can move parallel to the drum axis and, after each pass across the substrate, increment angularly so that the image on the substrate "grows" circumferentially. In both cases, after a complete scan by the beam, an image corresponding (positively or negatively) to the original document or picture will have been applied to the surface of the substrate.
Multiple imaging devices may be used to produce several lines of image spots simultaneously, with a corresponding increase in imaging speed. Regardless of the number of imaging devices used, their operation must be precisely controlled so that the discharges occur at the appropriate times to reach the intended dot locations on the printing surface. Each discharge source must be aligned with the substrate along longitudinal and lateral dimensions (corresponding to circumferential and axial directions in the case of drum imaging) at all points during a scan of the all candidate image points on the substrate, and, in the case of laser-based imaging, the beam must remain focused on the substrate for maximum energy-transfer efficiency.
Failure to maintain proper alignment along all relevant dimensions results in imaging inaccuracies and/or undesirable periodic artifacts that detract from the final image appearance. The consequences can be particularly acute in planographic printing contexts, since typical print jobs require sequential application of ink from several plates, each of which is vulnerable to image degradation if created digitally; the result is a cumulative aggregation of the imperfections associated with each plate. Laser imaging imposes perhaps the most demanding requirements, since adjustments along each of the three dimensions can result in introduction of distortions along the other dimensions.
The use of multiple imaging devices practically guarantees the occurrence of at least some printing artifacts at some resolutions. For example, even slight misalignment among imaging devices produces uneven "seams" between the regions or "zones" of the plate imaged by each device. These seams occur along the direction of imaging, and tend to be visible to the eye even when imaging takes place at high resolutions. Because the amount of misalignment that can produce visible artifacts is so slight, elimination of the problem through mechanical adjustment is unrealistic.
One electronic approach is disclosed in U.S. Pat. No. 5,453,777. In accordance with the teaching of that patent, compensation for imprecise device orientation (which may be exacerbated by focusing of the device) and "seam" artifacts is accomplished by, first, commencing imaging with the device having the greatest skew from proper orientation, and second, as a fine adjustment, repeating at least one column of image dots to produce intentional overlap between the imaging columns where a seam artifact would otherwise occur. Unfortunately, even this technique does not eliminate the artifacts in some circumstances.
Other approaches to avoiding seam artifacts have involved writing heads utilizing a series of discharge devices spaced closely together and along an axis perpendicular to the scanning axis. In these systems, the writing head makes successive passes across the recording medium, each pass applying a series of parallel rows of image spots. The problematic seams appear between successive passes of the writing head. Japanese Patent Application No. 60-107975, for example, discloses an ink-jet recording apparatus having a writing head comprising an array of 16 vertically aligned nozzles. A horizontal scan of the array, therefore, produces 16 parallel lines of image spots. To hide the seams between 16-line zones, the bottom eight nozzles of the array are selectively prevented from imaging during a scan; the array is then advanced vertically only halfway, so that the top eight nozzles overlie the eight lines just partially imaged by the bottom eight nozzles, and the top eight nozzles fill in the omitted image spots (as the bottom nozzles once again apply new image spots). It has been found, however, that the regular patterns of imaging locations reassigned from one nozzle set to the other can create their own visual artifacts.
Japanese Patent Application. No. 61-120578 illustrates a similar system that utilizes a less regular, apparently random blend pattern extending only marginally into adjacent imaging zones. This approach is likely to give unsatisfactory results in that a random pattern actually exacerbates the errors arising from device misalignment. These errors arise only at zone boundaries. Consequently, by perturbing the boundary only slightly, the errors will remain concentrated along a visible, vertically contiguous band. The effect, in other words, will not differ sufficiently from an unperturbed boundary. Moreover, the visual effect is worsened if the displacements occur at high frequency. In this case errors will be emphasized, since the rapid (and shallow) shifts in horizontal location only serves to make the errors more visible.